A number of observations support the existence of regulatory systems which assess endogenous protein/amino acid demand and alter food intake to meet this demand. Settings of negative protein balance lead to significant increases in food intake, resulting in excess energy intake and increased body adiposity as the animal attempts to meet the demand for protein. Thus regulatory systems governing protein intake are sufficient to override systems governing energy intake. Yet little is known about the mechanisms contributing to the regulation of protein intake or their relationship to energy balance and obesity. The long term goal of this project is to define the neuronal systems regulating food intake in response to varied protein balance. Results of recent studies indicate that amino acids, and specifically the branched-chain amino acid leucine, act locally in the hypothalamus to inhibit food intake and the expression of the orexigenic neuropeptides Npy and Agrp. At least two signaling pathways appear to mediate this response. One involves the intracellular signaling molecules mTOR (mammalian target of rapamycin) and AMP-activated protein kinase (AMPK); the second involves the local brain metabolism of leucine. The current project will 1) Test the hypothesis that reduced amino acid signaling within specific areas of the hypothalamus contributes to the hyperphagia induced by negative protein balance, and 2) Test the hypothesis that mTOR signaling, AMPK signaling, and/or branched-chain amino acid metabolism mediate the effect of amino acids on food intake and neuropeptide gene expression. Completion of these studies will dramatically increase our understanding of macronutrient composition and its impact on food intake and could lay the foundation for new interventions to control or reverse obesity. PUBLIC HEALTH RELEVANCE: The current project seeks to clarify the biological basis whereby the brain controls feeding behavior in response to variations in protein balance. This work is especially important considering that protein balance takes priority over energy balance, and thus tapping into this regulatory system will reveal novel approaches to regulate food intake and treat obesity. Protein intake is robustly regulated relative to other macronutrients, yet very little is known about the mechanisms by which the brain senses and regulates protein intake. Considering that this regulatory system is particularly important in periods of rapid growth and high protein demand and that many adolescents consume low quality diets, mechanisms regulating protein intake and their interaction with energy intake may have direct relevance to adolescent obesity.